It is known that metal-air batteries present remarkable characteristics which make them suitable for a number of important uses and that rechargeable zinc-air batteries are well known in the art. In one approach, the battery is recharged solely by application of electric current, however the zinc electrode (in practically relevant limited-electrolyte conditions), does not maintain a compact shape on repeated charge-discharge cycling, either forming zinc dendrites, which short out the cell, or the electrode undergoes zinc shape change, where the zinc tends to redistribute over the lower part of the plate with consequent capacity fading and stack deformation.
Zinc-air primary (non-rechargeable) cells are known in the art, but these cells have limited application. The use of a zinc electrode in a secondary (rechargeable) cell is also known, however, such cells present problems due to the formation of zinc dendrites during the recharging of the cell which will interfere with subsequent oxidation and reduction (discharging and recharging) of the zinc electrode and, thus, reduce the number of cycles during which the cell will deliver its full charging and discharging capabilities. Electrically rechargeable zinc-air cells and batteries incorporating a zinc anode together with a gas diffusion type air electrode cathode usually employ an alkaline electrolyte, where cycle life is often limited by the poor recharge characteristics of the zinc anode. The zinc plate discharge product (zinc oxide) is appreciably soluble in the alkaline electrolyte and tends not to reform the zinc active material on the plate during charge in a reproducible way. In many cases on charge the zinc reforms in an uncontrolled manner and grows towards the positive plate. (air cathode). This can cause cell or battery failure if the zinc shorts across to the positive plate (zinc dendrite failure), in some cases actually puncturing separator layers disposed between the plates or even puncturing the gas diffusion air electrode. Another common failure mode is anode shape change, where the zinc redistributes unevenly on the plate during charge/discharge cycling, causing deformation of the cell stack, and ultimate failure. These failure modes are not necessarily instantaneous in the life of the cell or battery, but can result in a steady falloff of performance (for instance capacity fading and voltage irregularities).
There have been various attempts to overcome this defect in electrically rechargeable zinc-air systems over the years, but none has provided a commercially feasible, long cycle life solution. In one approach the zinc is bonded in place using a polymer binder and additives that inhibit zinc dendrite formation or shape change on charge. In another approach the active zinc is mixed with a material (e.g. calcium oxide) that chemically traps the zinc plate discharge product (zinc oxide) in an insoluble form (in this case calcium zincate) before it can escape by partial dissolution into the alkaline electrolyte. Endurance can also be improved somewhat by incorporation of zinc dendrite-resistant multilayer separator systems that are ultimately punctured by dendrites and usually increase cell resistance. In a yet further approach the zinc active material is contoured and predistributed on the negative plate so as to allow for redistribution at the plate edges where shape change is particularly problematic, or an auxiliary electrode is positioned near the plate edges to dissolve off the excess zinc collecting there. In these and other approaches the occurrence of the problem is merely postponed not remedied.
Some examples of the various methods and schemes proposed to avoid or mitigate the zinc dendrite formation problem include, for example, Bronoel U.S. Pat. No. 4,517,258 which teaches the construction of a cell with a zinc negative electrode comprising spherically shaped particles which circulate in the electrolyte. The particles have a chemically resistant core and are coated with a conductive coating. The particles are charged and discharged by contact with a collector, e.g., they become coated with zinc when in contact with a collector more negative than that corresponding to the zinc deposit and in the presence of a zincate solution. The particles circulate in a KOH electrolyte which is pumped through the battery. The electrolyte drains off into a storage tank from which it is pumped back through the battery. The use of a floating negative electrode is said to inhibit the formation of zinc dendrites. However the circulating zinc particle slurry system adds considerable weight and parasitic load to the battery.
Iacovangelo et al, in an article entitled “Parametric Study of Zinc Deposition On Porous Carbon in a Flowing Electrolyte Cell”, published in the Journal of the Electrochemical Society, Volume 132 (1985), at page 851, describe the use of carbon foams as substrates for zinc electrodes in rechargeable zinc-bromine cells. The use of such a carbon foam provides an extended surface area upon which the reduced zinc may be deposited during recharging and reduction of the zinc, as well as a support surface which provides some degree of chemical inertness to the electrochemical reactions occurring in the cell. However, the formation of zinc dendrites on the surface of the foam during reduction of the zinc eventually blocks access to the inner surfaces of the foam and thereby eventually reduces the capacity of the electrode after a number of charging and discharging cycles.
There therefore remains a need to identify and produce a zinc-air cell, which is rechargeable, and does not suffer these limitations.